A gated field-emission device (or field emitter) is an electronic device that emits electrons when subjected to an electric field of sufficient strength. The electrons are extracted from an electron-emissive element by a gate electrode, and are subsequently collected at an anode spaced apart from the electron-emissive element and gate electrode. An area field emitter contains a group, often a very large group, of individual electron-emissive elements distributed across a supporting structure. Area field emitters are employed in CRTs of flat-panel televisions.
Referring to the drawings, FIG. 1 generally illustrates part of a conventional flat-panel CRT containing a field-emission backplate (or baseplate) structure 10 and an electron-receiving faceplate structure 12. Backplate structure 10 commonly consists of an electrically insulating backplate 14, an emitter (or base) electrode 16, an electrically insulating layer 18, a patterned gate electrode 20, and a conical electron-emissive element 22 situated in an aperture through insulating layer 18. The tip of electron-emissive element 22 is exposed through a corresponding opening in gate electrode 20. Emitter electrode 16 and electron-emissive element 22 together constitute a cathode for the illustrated part of the CRT. Faceplate structure 12 is formed with an electrically insulating faceplate 24, an anode 26, and a coating of phosphors 28.
Anode 26 is maintained at a positive voltage relative to cathode 16/22. The anode voltage is typically 300-700 volts for a conventional spacing of 100-200 .mu.m between structures 10 and 12. Because anode 26 is in contact with phosphors 28, the anode voltage is impressed on phosphors 28. When a suitable gate voltage is applied to gate electrode 20, electrons are emitted from electron-emissive element 22 at various values of off-normal emission angle .theta.. The emitted electrons follow parabolic trajectories indicated by lines 30 in FIG. 1 and impact on a target portion 28T of phosphors 28. The phosphors struck by the emitted electrons produce light of a selected color.
Phosphors 28 are part of a picture element ("pixel") that contains other phosphors (not shown) which emit light of different color than that produced by phosphors 28. Also, the pixel containing phosphors 28 adjoins one or more other pixels (not shown) in the CRT. If some of the electrons intended for phosphors 28 consistently strike other phosphors (in the same or another pixel), the image resolution and color purity are degraded.
The size of target phosphor portion 28T depends on the applied voltages and the geometric/dimensional characteristics of the CRT. Although the anode/phosphor voltage is typically 300-700 volts in the conventional flat-panel display of FIG. 1, power efficiency and phosphor lifetime are both considerably higher at a phosphor potential of 1,500-10,000 volts. However, increasing the anode/phosphor voltage to 1,500-10,000 volts in the CRT of FIG. 1 would require that the spacing between backplate structure 10 and faceplate structure 12 be much greater than the conventional value of 100-200 .mu.m. Increasing the inter-structure spacing to the value needed for a phosphor potential of 1,500-10,000 volts would, in turn, cause target phosphor portion 28T to become too large for a commercially viable flat-panel CRT display.
Focusing electrodes have been placed above the gate electrodes in field emitters to improve image resolution. For example, see U.S. Pat. Nos. 4,178,531, 5,070,282, and 5,235,244. Unfortunately, relatively complex processing at micrometer or submicrometer scale dimensions is usually needed to create a focusing electrode above the gate. It would be desirable to have a relatively simple gated field-emission structure that achieves high image resolution and color purity at high anode/phosphor voltage.